1. Field of the Invention
This invention relates to apparatus for protecting a synchronous machine from thermal overloads by limiting excitation and volts/hertz. In particular, the invention is directed to providing such protection in response to closely spaced events using a microprocessor-based limiter.
2. Background Information
Large synchronous machines, such as generators used in power generation systems and the machines used to adjust the power factor in such systems, have a stator winding connected to the power system and a rotor, or field, winding which is excited by a dc exciter through slip rings. The rotating dc field produced by turning of the rotor generates an ac field in the stator winding. A voltage regulator attempts to regulate the stator voltage to a desired set point value in a feedback loop which regulates the field current. In the case of a small generator, or even a large generator in a very large power system, terminal voltage of a synchronous machine is set by the power system. If the system voltage becomes depressed, the voltage regulator on the machine attempts to raise the voltage by increasing the field current. This leads to overheating of the rotor and damage to the machine.
Conventionally, synchronous machines are protected by several limiters. A maximum excitation limiter protects the synchronous generating equipment against excessive field current, i.e., excitation. This limiter is typically part of, and acts through, the generator's automatic voltage regulation equipment. Under certain power system fault conditions, or as a result of malfunctions in auxiliary devices, the automatic voltage regulator may attempt to increase the generator field current to a point where rotor heating endangers the machine. Maximum excitation limiters monitor field current and will act to reduce field current under such circumstances; essentially overriding any other attempts to maintain or increase excitation. A time delay related to the degree of excessive excitation is applied before limiting action actually occurs.
Synchronous generating equipment is also routinely protected against excessive rotor flux densities by equipment known as a volts per hertz limiter. This limiter is typically also a subsystem of, and acts through, the generator's automatic voltage regulation equipment. During certain power system disturbances, or as a result of operator actions or malfunctions in auxiliary devices, the automatic voltage regulator may attempt to increase the generator excitation to a point where stator heating endangers the machine. Reduced speed (under frequency) operation requires that the machine excitation be decreased proportionally. Attempting to maintain nominal terminal voltage at under frequency conditions can result in excessive magnetic flux densities in both the generator stator core and in the core of the unit transformer. Excessive flux density results in hysteresis and eddy current losses and overheating of the magnetic material. Such situations may also occur during machine start-up where prolonged periods of underspeed operation are common. Volts Per Hertz (V/Hz) limiters will act to reduce excitation under such circumstances, essentially overriding any other attempts to maintain or increase excitation. Again, a time delay related to the degree of excessive excitation is applied before limiting action actually occurs. The degree of limiting action is proportional to a volts per hertz ratio in excess of a safe level.
These maximum excitation limiters and V/Hz limiters are effective at limiting excitation, but once the excitation returns to safe levels, the limiting action ceases. If the excitation again exceeds safe levels, the limiting algorithms simply reinitiate all time delay cycles. No knowledge of any preceding events is recognized by these known systems. In situations where such events are widely separated in time, the rotor or the magnetic material will have sufficient time to cool. However, if such events occur too closely spaced in time, the rotor or the magnetic material may not return to its normal temperature before the next event begins. The traditional implementation of these limiters reinitializes the delay timers at the end of each event with no allowance for the fact that the rotor or magnetic material is entering the next event at an already elevated temperature. Under these circumstances, the limiting action will not have the needed effect.
There is a need therefore, for improved protection of a synchronous machine. More particularly, there is a need for apparatus for protecting a synchronous machine from events closely spaced in time which can cause thermal damage to the machine.
Specifically, there is a need for improved apparatus for protecting a synchronous machine from over excitation and excess V/Hz events which occur close enough in time that the machine has not had time to cool to normal temperature before the next event occurs.
There is also a need for such improved apparatus which can operate through the existing voltage regulation system for synchronous machines.
There is a further need for such an improved apparatus in which the protection can accommodate for changes in the cooling rate of the machine following an event.